This invention relates to a filter device for use in diagnostic X-ray systems. The new filter device is applicable to various X-ray systems but will be illustrated in connection with a computed projection radiography system.
A computed projection radiography (CPR) system produces conventional radiographic type images using a computer to process the X-ray attenuation data for the images instead of using a film. In the CPR method, a thin fan-shaped beam of X-rays is formed. Typically, the beam is collimated to a thickness in the range of 1.5 to 2.0 mm. The beam is pulsed on and off at a rate of typically 60 times per second. The patient being examined is moved in linear increments in a longitudinal direction that is perpendicular to the plane of the pulsed fan-shaped X-ray beam. A multicell X-ray detector is located for intercepting the thin differentially attenuated X-ray beam as it emerges from the body. The detector cells each intercept a small bundle of rays and each produces an analog signal representative of the X-ray beam attenuation along its ray path through the body for each X-ray pulse. The analog attenuation data obtained during each pulse from each body slice or view is subjected to an appropriate algorithm in a computer that yields pixel signals which, when assembled in an array corresponding with the width and length of the body portion that has been scanned can be used to reconstruct a two-dimensional radiographic image for display on a television monitor, for example. It should be noted that the body can be moved through the beam or the beam can be scanned over the body.
Use of a thin X-ray beam as in CPR reduces the effects of radiation scattering within the body so image resolution is improved. In ordinary projection radiography where the image is recorded on film and the X-ray beam is divergent in orthogonal directions and wide enough to cover the entire field of view at one time, radiation is scattered by volume elements in the body from one ray path to another or, in other words, it is scattered from substantially any part of the field to all other parts of the field. This results in less than optimum image contrast and resolution.
The more advanced CPR system in which the new filter device disclosed herein will be illustrated provides at least a pair of X-ray pulses for each slice or longitudinal increment of the body being scanned. One X-ray pulse in a pair has higher energy than that of the other. The second pulse in a pair can start just as soon as the first one ends so that both pulse beams pass through the same slice of the body. The bouble pulse scan is continued over the anatomical region of interest. A separate set of attenuation data for each slice of the body, that is, for each high and low energy X-ray pulse is acquired on a line-by-line basis and the data for the pulses are switched to different memories as they are acquired for being processed by the computer. This would permit reconstruction of two images which are substantially spatially coincident. After the computer reconstructs the images they could be displayed individually. Usually, however, the picture element data for the two independent images is weighted and then the data for one image is subtracted from the other to produce a matrix of picture element data that results in a displayed image wherein anatomy that is obscured in the individual images is easily visualized. The data for the individual images can also be processed by the computer to produce edge enhancement, noise suppression, gamma correction and accomplish window and level control of intensities that are not otherwise achievable with radiographic film.
As is known, for performing subtraction CPR, it would be desirable for the low and high energy beams to be monoenergetic or fall within very narrow spectral bands. However, when a particular high or low kilovoltage is applied between the anode and cathode of the X-ray tube the resulting X-ray beam will be comprised of photons having a distribution of energies. The customary way of narrowing the spectral band is to insert filter elements in the X-ray beam. Thus, for subtraction CPR, during the high energy X-ray beam pulse, a filter that removes photons having an energy significantly below the selected high energy is inserted in the beam. During the low energy pulse, the filter that takes out photons having an energy above a predetermined level is inserted in the beam.
The concept of obtaining X-ray beams at different energy levels by rotating two different kinds of filters across the path of a polyenergetic X-ray beam, rather than successive beams at different energies, so that beams of different spectral bands can be obtained in close succession is known in the prior art. U.S. Pat. No. 4,029,963, particularly in FIG. 2, illustrates a rotating filter device comprising a disk having diametrically opposite apertures in which filters that pass different spectral bands are located. The system uses the X-ray detector output to trigger the switch which directs data corresponding to the different X-ray energies to different memories. A disadvantage of this is that signals at one energy level are still persisting while signals at the other energy level have begun to occur or are about to begin occurring. Little if any time is available for the detector elements to deionize or clear. The system is not appropriate for the preferred case where the X-ray tube itself is pulsed at two different voltage levels as in the CPR system described herein. In addition, with prior art filter devices it would be difficult, if not impossible, to have a X-ray pulses occur when the respective filters are in the identical angular position during each revolution.